Magnesium is a lightweight, corrosion resistant, high-strength metal extensively used in the airspace industry. On a volumetric weight basis, it is 80% lighter than steel and 33% lighter than aluminum. Magnesium alloys developed for the aerospace and automotive industries have a strength to weight ratio greater than that of steel, aluminum or titanium. Cast magnesium components are easily machined to produce high-strength lightweight components that enable weight reduction and increased fuel efficiency.

Magnesium has a relatively low melting point and a thermal expansion coefficient of approximately 25 to 26.9 (10-6 mm/(mm °C)). Most magnesium alloys are dimensionally stable to about 200° F, although thorium, cerium and zirconium alloying elements are added to increase its yield strength at working temperatures of up to 700° F.

Magnesium also has a high oxidation potential and surface contaminants and oxides are known to be problematic. To produce a sound weld, precautions must be taken to account for its low melting point, high thermal expansion coefficient and high oxidation potential.

Preheating

Excess heat input into a magnesium casting during the welding process can lead to cracks and embrittlement as thermal expansion leads to thermal stress in the heat affected zone. Excess heat can also lead to strength reductions as the low melting point allows grain boundaries to migrate at moderate temperatures.

To overcome these physical limitations, name="_Hlk27334440">magnesium welds are completed with low and well-controlled power input, preferably at high welding speeds. To accomplish these tasks when dealing with thicker workpieces, preheating the workpiece to a temperature in the range of 200 to 300° C can provide several benefits.

Figure 1: Magnesium welds are completed with low and well controlled power input at high welding speeds. Source: U.S. Air Force photo by Airman 1st Class Dillon J. Audit

Preheating helps alleviate thermal stress during the welding process, thereby reducing the potential for cracking in the heat affected zone. It also facilitates greater welding speeds, which helps reduce heat input into the workpiece. Preheating should be completed in an oven with a controlled atmosphere to reduce oxidation potential.

Surface preparations

To eliminate any amount of porosity in the weld, proper cleaning is paramount. During shipping and handling, an oil coating or chrome finish is typically applied. Cast components also tend to soak up oil. These coatings, impregnated oils, surface metal oxides and any other foreign substances must be removed before welding. Even burs or magnesium dust can be problematic as magnesium will ignite and burn when heated in an open atmosphere. Joints should be smooth and free of loose pieces or cavities that might contain contaminants before welding.

Chemical cleaning is generally preferred although mechanical cleaning can also be performed. Chemical cleaning may be accompanied by hazardous or flammable vapors and proper personnel protective equipment along with proper engineering controls should be selected based on cleaning methods to reduce exposure potential. Popular methods of chemical cleaning include use of chlorinated solvents, dry cleaning solvents of mineral spirits.

Mechanical cleaning is accomplished with 160 and 240 grit abrasives. Aluminum oxide abrasive cloths or stainless-steel wool are acceptable for cleaning magnesium alloys, although steel brushes should be avoided as residual steel particles can lead to galvanic corrosion.

Conclusion

Increased use of cast magnesium alloy components in the automotive sector and continued use in the aerospace industry is expected as it aids in accomplishing weight reductions to meet future fuel efficiency requirements. Repair welds on these cast magnesium components can be accomplished successfully with proper setup. To maintain strength integrity of the base metal and produce a weld of sufficient strength, proper surface preparation techniques and pre-heating requirements should be followed as prescribed by welding methods that are specific to the part geometry, operation and alloy to be joined.

Be sure to read Part 2 of this series to learn more about shielding requirements and filler alloys for magnesium welding.